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Scatter radiation

Ch.15 and Ch.18

QuestionAnswer
Scatter radiation is the primary result of Compton interaction
Incoming x-ray photon (interacts with the outer shell electron) loses energy and changes direction (scatter). Compton
The patient is the Greatest source of scatter
___ is not useful and exposes IR and degrades images Scatter
Collimating and using a grid reduces Scatter
4 factors that affect amount of scatter radiation exiting the patient is KVP, Thickness of tissue, Volume of tissue irradiated (size of xray beam) and Z#
Higher Z#, ____ PE, and ____ scatter More, Less
The amount of scatter exiting the patient depends on The KVP selected
As KVP is increased _____ interactions occur, and ____ photons pass through patient to IR. Less; More
As KVP increases: Contrast _____ Scatter _____ and IR exposure _____ Contrast Decreases, Scatter Increases, and IR exposure increases.
The larger the xray beam field size The greater the amount of scatter radiation produced.
The larger the body part, the more tissues to interact with photons, so... More scatter is created
We can decrease/reduce patient thickness by using a _____ or by placing the patient on their stomach Compression Bands
A lower Z# _____ PE, and _____ scatter Less PE, More Scatter
Collimation is Beam Restriction
Beam restriction serves 2 purposes: Limiting patient exposure, reducing amount of scatter radiation
Limiting the xray beam field size is accomplished with a Beam - restricting device
As beam restriction or collimation increases, Field size ____ and patient dose _____ Decreases; decreases
Decreasing patient dose, beam restricting devices also reduce scatter produced within the patient, reduce scatter to image receptor, so it also increases radiographic contrast
Collimation ______ , density ______, contrast improves Increases, decreases
mAs Density
KVP Contrast
Collimation requires an increase as much as __________ of the mAs to compensate for the loss of density that occurs because of collimation 30% - 50%
Flat piece of lead (diaphragm) that has a hole (aperture) in it. Aperture Diaphragm
Shaped differently, but work the same. They slide onto the tube below the window Cones and Cyclinder
Types of beam - restricting devices Aperture Diaphragm, Cones, Cyclinders, collimators
Automatically collimates the primary beam to the shape and size of the image receptor Positive Beam Limiting Devices (PBL)
The field size should never be larger than the IR / Cassette
The most common beam restrictor in radiography is Collimator
A collimator has sets of _____ at right angles to one another that move in opposing pairs Lead Shutters
The lead shutters have 2 purposes. Regulate field size and reduce penumbra
A geometric unsharpness around the periphery of the image, also called edge unsharpness Penumbra
Which lead shutter reduces penumbra Bottom Lead shutter
Which lead shutter help reduce the amount of off-focus radiation? Upper Lead shutter
The collimator is also a light source. The light field is provided by a ______ that is mounted in the path of the x-ray beam at a 45 degree angle. Mirror
Who invented the radiographic grid in 1913? Gustave Bucky
Grids do what? Absorb scatter and improve contrast
Grids require Additional mAs, resulting in higher patient dose.
Grids need to be applied when Anatomic part is thicker than 10 cm and KVP is above 60.
Scatter decreases Contrast
What contains lead strip or lines that have a precise height, thickness, and space between them and are radiopaque Grids
Radiopaque lead lines are dense, absorb scatter, high z#
Radiolucent interspace material separates the lead lines: typically made of Aluminum
Radiopaque Blocks radiation
Radiolucent Allows radiation through
_____ can be described by grid frequency and grid ratio Grid Construction
The numbers of lead lines per unit length, in inches, centimeters or both. Grid Frequency
The ratio of the height of the lead strips to the distance between the strips Grid Ratio
Grid Ratio formula? H/D Height over Distance
As Grid ratio increases, scatter cleanup improves and contrast Increases
Removes more scatter, improve contrast, and has less distance between lead strips and doesn't allow scatter to pass to IR High Ratio Grid
What are the two types of grid patterns? Linear and Cross - Hatched
Has lead lines that run at a right angle to one another Cross hatched grid
Has lead lines that run in only one direction. Linear pattern grid
Which is most popular because they allow angulation of the xray tube Linear pattern grids
Which grid removes more scatter and are limited because the xray tube cannot be angled Cross hatched grid
What are the two types of grid focus? Parallel (non-focused) and Focused
Has lead lines that run parallel to one another Parallel (non-focused) Grid
Has lead lines that are angled or canted to match the angle of divergence of the primary beam Focused Grid
______ allows more transmitted photons to reach the IR Focused Grid
For a grid to be properly focused, the x-ray tube must be located along the Convergent line
Sometimes called the grid radius Focal distance
The convergent and convergent point are important because they determine the Focal distance of a focused grid
What are the two types of grids? Stationary and Moving grids
Matches the size of the cassette and is used by placing it on top of the image receptor Wafer grid
An IR that has a grid permanently mounted to its surface Grid Cassette
Permanently mounted grid and allows for the IR to slide in behind it Grid Cap
What are the wafer grid, grid cassette, and cap grid? Stationary Grids
Move during the xray exposure in a lateral direction and blurs the grids lines Moving grids
Moving or reciprocating grids are part of the buckynand are called Potter Bucky Diaphragm
The purpose of the grid is to increase Contrast
Grids with higher ratios increase contrast because higher frequency cleans up more scatter
As grid ratio Increases, density ______ , contrast increases. Decreases
The disadvantages of grids are The increase patient dose and possibility of grid cut off
_______ is a decrease in the number of transmitted photons that reach the IR because of some misalignment of the grid Grid cutoff
What are the 5 grid errors Off level, off center, off focus, upside down, the Moire effect.
When the tube is angled across the long axis of the grid strips. Improper tube or grid positioning. Most common occurs with stationary grids being used for mobile exams or decubitus views Off-level grid error
The x-ray tube has to be centered along the central axis of a focused grid. If the CR is off center, it wont correspond to the grid and a result of decreased exposure occurs Off center grid error
When a grid used at a distance other than specified as a focal range. Will have grid cut off along the peripheral edges of the image. Higher grid ratio requires a greater positioning accuracy. Off Focus Grid Error
Results in grid cut off on the peripheral side of images Upside-down Grid Error
Occurs ONLY with digital IR systems, when the grid lines are captured and scanned parallel to the scan line in the imaging plate readers. Most plates scanned across the short axis of the plate Moire Effect Grid Error
As grid ratio increases: Contrast, patient dose, and likelihood of grid cut off _______ Increases
Linear grid pattern, Focused grid, mid ratio (8:1 to 12:1) and has a focal range of SID of 40 or 72 inches is a Typical Grid
This technique is based on the simple concept that much of the scatter will be miss the IR if there is increased distance between the patient and the IR (increased OID) Air gap Technique
The air gap technique, the greater the gap, the greater the reduction in ____ scatter reaching the IR
Air gap is limited because the necessary IOD results in ______ recorded detail Decreased
The air gap is an alternative to... using a grid to control scatter reaching the IR
The air gap technique results in patient dose that is the same as or slightly less than, using a comparable grid. Because a grid absorbs some of the transmitted photons, where the air gap technique does not.
GCF= mAs with the grid / mAs without the grid
Created by: deleted user
 

 



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